Several publications and patent documents are cited throughout the specification in order to describe the state of the art to which this invention pertains. Each of these citations is incorporated herein by reference as though set forth in full.
Over the past two decades, the recognition of specific molecular patterns has been shown to play a central role in the immune responses of plants and animals (Boller and Felix (2009) Annu Rev Plant Biol., 60:379-406; Ronald and Beutler (2010) Science 330:1061-1064; Pieterse et al. (2012) Annu Rev Cell Dev Biol., 28:489-521). Plants and animals have been shown to possess pattern recognition receptors that serve to detect several different molecular signatures associated with specific classes of microbes. For example, Arabidopsis recognize bacteria using specific pattern recognition receptors (PRRs) for flagellin, lipopolysaccharide, peptidoglycan, and other pathogen-associated molecular patterns (PAMPs). Because not only pathogenic microbes are recognized in this manner, these molecular signatures are also referred to by the more general term, Microbe-Associated Molecular Patterns (MAMPs; (Bittel and Robatzek (2007) Curr. Opin. Plant Biol., 10:335-341). MAMPs include carbohydrates, (glyco)-proteins, lipids, peptides, and sterols (Boller, T. (1995) Annu Rev Plant Phys., 46:189-214; Ebel and Mithofer (1998) Planta 206:335-348; Nurnberger et al. (2004) Immunol Rev., 198:249-266). MAMPs/PAMPs are perceived at low concentrations and act as inducers of defense responses (Boller, T. (1995) Annu Rev Plant Phys., 46:189-214; Ebel and Mithofer (1998) Planta 206:335-348). Additionally, PAMP perception can lead to long-term sensitization of plants, resulting in more rapid and/or more intense activation of future defense responses, which can lead to enhanced resistance to both biotic and abiotic stresses (Conrath et al. (2006) Molecular Plant-Microbe Interactions 19:1062-1071).
Similar defense responses can be triggered by molecular species originating from the plant itself, so-called damage-associated molecular patterns (DAMPs; Bianchi, M. E. (2007) J. Leukocyte Biol., 81:1-5), which, for example, would result from herbivory by insects. In contrast, there are no known conserved insect- or nematode-associated molecular patterns that are recognized by plants, although a few species- or genus-specific families of lipid-derived small molecules from insect oral secretions have been shown to trigger plant defense responses (Schmelz et al. (2009) PNAS 106:653-657; Schroder, F. (1998) Angewandte Chemie-Intl. Ed., 37:1213-1216). In addition, oral secretions (OS) from feeding insects contain Herbivore-Associated Elicitors (HAE), which are also called Herbivore-Associated Molecular Patterns (HAMPs). This latter term covers all the herbivore-derived signaling compounds that might come into contact with a particular host plant and elicit defense responses (Bonaventure et al. (2011) Trends Plant Sci., 16, 294-299; Mithofer and Boland (2008) Plant Physiol., 146:825-831). Host perception of MAMPs, DAMPS, and HAMPs has been shown to involve shared signal transduction mechanisms, including activation of MAPKs, generation of reactive oxygen species (ROS), and activation of salicylic acid (SA)- and jasmonic acid (JA)-signaling pathways (Bonaventure et al. (2011) Trends Plant Sci., 16, 294-299; Kallenbach et al. (2010) Plant Physiol., 152:96-106; Asai et al. (2002) Nature 415:977-983; Pieterse et al. (2012) Annu. Rev. Cell Dev. Biol., 28:489-521; Robert-Seilaniantz et al. (2011) Annu. Rev. Phytopathol., 49:317-343).
Nematodes are arguably the most numerous animals on earth. They are ubiquitous in soil and parasitize most plants and animals, and as a result cause agricultural damage of more than $100 B annually worldwide (Blumenthal and Davis (2004) Nat Genet., 36:1246-1247; Mitkowski et al. (2003) Nematology 5:77-83). Plants perceive the presence of nematodes and respond by activating defense pathways. For example, root knot nematodes and rhizobial Nod factors elicit common signal transduction events in Lotus japonicus (Weerasinghe et al. (2005) Proc Natl Acad Sci., 102:3147-3152), and prior inoculation with avirulent (host-incompatible) Meloidogyne incognita in a tomato split-root assay reduced susceptibility to virulent (host-compatible) M. hapla (Ogallo et al. (1995) J Nematol., 27:441-447). Antagonistic effects of entomopathogenic nematodes on plant-parasitic nematodes (Molina et al. (2007) J Nematol., 39:338-342) also may be due to induction of plant defenses, such as expression of pathogenesis-related protein-1 (PR-1) and increased catalase and peroxidase activity, not only in the roots, but also in the leaves (Jagdale et al. (2009) J. Nematology 41:341-341; Jagdale et al. (2009) Biol Control 51:102-109). However, the nature of the nematode-derived signal(s) and the subsequent signaling pathway(s) leading to defense responses have remained unclear.
Ascarosides represent an evolutionarily conserved family of nematode-derived small molecules that serve essential functions in regulating development and social behaviors (Choe et al. (2012) Curr. Biol., 22:772-780; Pungaliya et al. (2009) Proc Natl Acad Sci., 106:7708-7713; Srinivasan et al. (2008) Nature 454:1115-1118; Srinivasan et al. (2012) PLoS Biol 10:e1001237; von Reuss et al. (2012) J Am Chem Soc., 134:1817-1824; Butcher et al. (2007) Nat. Chem. Biol., 3:420-422; Golden et al. (1982) Science 218:578-580; Jeong et al. (2005) Nature 433:541-545; Kaplan et al. (2012) PLoS ONE 7:e38735; Ludewig et al. (2013) WormBook, 1-22; Noguez, Jet al. (2012) ACS Chem Biol 7:961-966). Ascarosides are glycosides of the dideoxysugar ascarylose that carry a fatty acid-derived lipophilic side chain and have been identified exclusively from nematodes. For example, in the model organisms Caenorhabditis elegans, and Pristionchus pacificus as well as in the insect parasitic nematode Heterorhabditis bacteriophora, ascarosides regulate entry into stress resistant dispersal or infective larval stages (Bose et al. (2012) Angew Chem Int Ed Engl., 51:12438-12443; Noguez et al. (2012) ACS Chem Biol., 7:961-966; Pungaliya et al. (2009) Proc Natl Acad Sci., 106:7708-7713). Whereas some nematode ascarosides (NAs) are broadly produced among different nematode species, other NAs are highly species-specific or are associated primarily with a specific ecology. For example, the NA ascr#9 is particularly common among entomopathogenic (insect-parasitic) nematodes (Choe et al. (2012) Curr Biol., 22:772-780), whereas the longer-chained ascr#18 is produced by several species of the plant-parasitic genus Meloidogyne. Different structural variants are often associated with starkly different activity profiles, and biological activity is frequently observed at very low concentrations (Bose et al. (2012) Angew Chem Int Ed Engl., 51:12438-12443; Pungaliya et al. (2009) Proc Natl Acad Sci., 106:7708-7713; von Reuss et al. (2012) J Am Chem Soc., 134:1817-1824; Izrayelit et al. (2012) ACS Chem Biol., 7:1321-1325).
More than 200 different NA structures from over 20 different nematode species have been identified, demonstrating that NAs are widely distributed in the nematode phylum, including both human-parasitic and plant-parasitic nematodes (Choe et al. (2012) Curr Biol., 22:772-780; von Reuss et al. (2012) J Am Chem Soc., 134:1817-1824; Bose et al. (2012) Angew Chem Int Ed Engl., 51:12438-12443). These results indicated that NAs represent a highly conserved molecular signature of nematodes. Based on these results, it seemed possible that NAs are also perceived by the organisms that nematodes interact with, including their plant and animal hosts as well as nematode-associated microorganisms.